The present invention relates to a method and an apparatus for mixing a fluid with a liquid medium. Specifically, the present invention discloses a method and an apparatus by means of which a fluid is mixed with a liquid medium such that the mixer apparatus need not necessarily be provided with a drive motor at all, in other words at least a part of the power needed to drive the rotor is arranged by using a fluid to rotate the rotor of the mixer.
The majority of prior art mixing devices are either static mixers, which have no moving parts, or dynamic mixers, which have a rotor rotating in a mixing chamber, the rotor being driven by means of a drive unit, normally an electric motor. From time to time such dynamic mixing devices appear on the market that have no drive unit. What makes the use of such devices possible is that sometimes the kinetic energy of a medium entering the mixing chamber is utilised, by means of a specifically designed rotor, to rotate the mixer rotor.
An example of such mixing devices has been disclosed in U.S. Pat. No. 6,193,406. The US-patent discusses a method and an apparatus for mixing different chemicals, both liquid and gaseous, or steam into pulp suspension in the wood processing industry. In the method and the apparatus according to the patent, the pulp suspension and the fluid medium are fed into a mixer casing, mixed therein by means of a freely rotatable mixer rotor and removed from the casing. The freely rotatable mixer rotor provided with mixing blades is placed within the casing and made to rotate by means of incoming flow of pulp suspension being in contact with the mixing blades of the rotor. The fluid medium i.e. the medium to be mixed with the fiber suspension is introduced into the pulp flow either upstream of the mixer casing or directly into the mixer casing in the effective area of the rotating rotor. Thus, both the mixer rotor and the mixing chamber have been designed such that the fiber suspension entering the mixing chamber rotates the rotor. In other words, the mixing function takes place simultaneously and in the same cavity as the rotating function.
However, arranging the rotating of the rotor in the same cavity with the mixing of the two or more media brings about problems, as the requirements set for the cavity by the mixing function are different from the requirements set by the rotating function. Since the rotating of the rotor should be affected by as low use of energy as possible, and since the mixing should be performed as efficiently as possible, it is practically impossible to develop an apparatus that could fulfil simultaneously the requirements of both tasks.
Another problem of this type of a mixer appears when a gaseous substance is intended to be mixed with a liquid. Normally, when mixing gas and liquid, the rotor has to be designed such that the rotor prevents the accumulation of gas in the center of the mixing chamber. This is achieved by arranging the rotor to affect a strong turbulence field in the mixing chamber such that powerful flow components in radial direction are created. However, to be able to perform the above described function the design of the rotor vanes is far from ideal in view of rotating the rotor. Thus, if a major task of the rotor vanes is to rotate the rotor, the design of the rotor vanes is such that the radial force field they are able to create is substantially weak, which results in that the rotor rotates at least partially in a gas bubble, and, as a result, the rotor is not able to mix the gaseous substance efficiently in the liquid.